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Abstract:

Disclosed is a novel cooperative communication strategy jointly using
symbol-level random network coding and hierarchical modulation in order
to effectively minimize packet error rate in error prone wireless
networks. The source (or sender) broadcasts random network coded symbols
with hierarchical modulation to the relays and the destination (or
receiver). In following time slots, the relays, which have successfully
decoded the original packet, transmit additional random network coded
symbols to the destination. By applying the present disclosure into a
multi-hop relay consumer device network, which comprises a set of
consumer devices, error free transmission with high efficiency can be
achieved.

Claims:

1. A method for transmitting data in wireless communication system, the
method comprising: dividing an input bit stream into segments; adding
error detection code into each segments of the divided input bit stream
in order to generate a packet; dividing the generated packet into a
plurality of blocks with fixed size; coding each of the plurality of
blocks using a random linear coding in order to generate coded block
bits; mapping the coded block bits to one of a hierarchical modulation
bit positions, wherein the hierarchical modulation bit positions are a
group of bits representing a priority class; and transmitting the
plurality of modulated symbols.

2. The method of claim 1, wherein the priority class is at least one of a
high priority, a medium priority, and a low priority.

3. The method of claim 1, wherein the all steps are implemented in a
multi-hop relay networks.

4. The method of claim 1, wherein the coded block bits in a coded block
are rearranged according to a bit error probability.

5. A method for receiving data in wireless communication system, the
method comprising: receiving a plurality of modulated symbols; generating
coded block bits by selecting bits from the hierarchical modulation bit
positions of the received plurality of modulated symbols, wherein the
hierarchical modulation bit positions is a group of bit representing the
priority class; and performing a random linear decoding with the coded
blocks to restore a packet.

6. The method of claim 5, wherein the priority class is at least one of a
high priority, a medium priority, and a low priority.

7. The method of claim 5, wherein the all steps are implemented in a
multi-hop relay networks.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a wireless communication system
and a terminal providing a wireless communication service and to a method
of jointly using symbol-level random network coding and hierarchical
modulation in order to effectively minimize packet error rate in error
prone wireless networks.

BACKGROUND ART

[0002] Multi-channel wireless networks represent a direction that most
future 4G state-of-the-art wireless communication standards evolve
towards, including IEEE 802.16 Wi-MAX and 3GPP Long Term Evolution (LTE).
In both Wi-MAX and LTE, Orthogonal Frequency Division Multiple Access
(OFDMA) is used at the physical layer. OFDMA uses a large number of
orthogonal subcarriers to maximize spectral efficiency, and assigns
different subsets to different users to achieve multiple accesses. It is
common knowledge that errors are inherently present in unreliable
wireless channels. The important challenge in designing error control
protocols in the MAC or physical layer is to effectively maximize
achievable throughput in various transmission scenarios in wireless
networks even when unpredictable and time-varying errors exist.

[0004] Therefore, in order to overcome the wireless channel impairment,
this disclosure may propose the coded cooperation in relay communication.
In fact, the code cooperation in relay communication shows that the
simple strategy using distributed channel coding in cooperative
communication increases system performance. When hierarchical modulation
is considered with the coded cooperation (denoted as "Co-HM" hereafter),
additional system performance improvement and simple transmission
strategy can be obtained. Further, this disclosure proposes a joint
symbol-level random network coding and hierarchical modulation scheme in
a relay communication (denoted as "Co-NC").

[0005] To achieve these and other advantages and in accordance with the
purpose of the present invention, as embodied and broadly described
herein, there is provided a method for transmitting data in wireless
communication system, the method comprising: dividing an input bit stream
into segments; adding error detection code into each segments of the
divided input bit stream in order to generate a packet; dividing the
generated packet into a plurality of blocks with fixed size; coding each
of the plurality of blocks using a random linear coding in order to
generate coded block bits; mapping the coded block bits to one of a
hierarchical modulation bit positions, wherein the hierarchical
modulation bit positions are a group of bits representing a priority
class; and transmitting the plurality of modulated symbols.

[0006] To achieve these and other advantages and in accordance with the
purpose of the present invention, as embodied and broadly described
herein, there is also provided a method for receiving data in wireless
communication system, the method comprising: receiving a plurality of
modulated symbols; generating coded block bits by selecting bits from the
hierarchical modulation bit positions of the received plurality of
modulated symbols, wherein the hierarchical modulation bit positions is a
group of bit representing the priority class; and performing a random
linear decoding with the coded blocks to restore a packet.

[0007] The foregoing and other objects, features, aspects and advantages
of the present invention will become more apparent from the following
detailed description of the present invention when taken in conjunction
with the accompanying drawings.

[0014] One aspect of this disclosure relates to the recognition by the
present inventors about the problems of the related art as described
above, and further explained hereafter. Based upon this recognition, the
features of this disclosure have been developed.

[0015] Although this disclosure is shown to be implemented in a specific
mobile communication system, this disclosure may also be applied to other
communication systems operating in conformity with different standards
and specifications.

[0016] Hereinafter, description of structures and operations of the
preferred embodiments according to the present invention will be given
with reference to the accompanying drawings.

[0017] With respect to the objective of maximizing the resource usage
performance, network coding has been originally proposed in information
theory and has since emerged as one of the most promising information
theoretic approaches to improve throughput. For example, a MIXIT, which
utilize a protocol for cooperative packet recovery by performing random
network coding across correct symbols and opportunistic routing on groups
of correctly received symbols, has been proposed. A MIXIT system improves
a throughput of wireless mesh networks. Instead of insisting on
forwarding only correct packets, the MIXIT routers use physical layer
hints to make their best guess about which bits in a corrupted packet are
likely correct and forward them to the destination. Even though this
approach inevitably lets erroneous bits through, it has been proved that
it achieves high throughput without compromising end-to-end reliability.

[0018] The core component of MIXIT is a novel network code that operates
on small groups of bits, called symbols. It allows the nodes to
opportunistically route correctly-received bits to their destination with
low overhead. MIXIT's network code also incorporates an end-to-end error
correction component that the destination uses to correct any errors that
might seep through.

[0019] Further, there is proposal that pre-coded transmission scheme using
random network coding outperforms the frequency diversity. Further, a MAC
layer Random Network Coding (MRNC) has been introduced to avoid the
overhead problems incurred by HARQ in the application of Wi-MAX. The
random network coding can be applied in practical multi-hop wireless
networks.

[0020] Since modulation selection scheme affects the system performance in
a wireless communication network, a sophisticated modulation selection
scheme is required for optimal adaptive modulation scheme. By employing
Co-HM (i.e., hierarchical modulation is considered with the coded
cooperation), modulation selection scheme can be simplified. However, the
Co-HM scheme cannot take full advantage of the benefits of relay
communication for the transmission of one encoding blocks due to code
rate limitation. The code rate of the all received blocks cannot be
decreased less than the mother code rate used for encoding of the
transmitted block in the sender. For example, when a mother code of rate
1/2 is used in the sender for certain encoding blocks, regardless of how
many relays decode the transmitted encoding blocks and transmit
re-encoded redundancy, the receiver can only receive the codes with rate
1/2. To overcome the inefficiency of Co-HM, this disclosure proposes a
scheme to take advantage of random network coding and coded cooperation
with hierarchical modulation. And, this symbol-level random network coded
communication strategy with hierarchical modulation is a novel technique
that achieves an error free transmission with high efficiency.

[0021] As aforementioned, this disclosure proposes a joint symbol-level
random network coding and hierarchical modulation scheme in a relay
communication (denoted as "Co-NC"). In fact, simulation results for
Additive White Gaussian Noise (AWGN) channel show that the proposed
scheme, Co-NC outperforms the performance of Co-HM in terms of Packet
Error Rate (PER).

[0022] A more detailed description of the present disclosure will be given
as following. A hierarchical modulation is supported in various standards
including Digital Video Broadcasting (DVB). The hierarchical modulation
takes in two streams with differing service requirements and transmits
both streams over the same radio frequency (RF) channel. In the
hierarchical modulation, the two most significant bits of the 16 or 64
Quadrature Amplitude Modulation (QAM) symbol convey high priority service
data which mapped with Quadrature Phase Shift Keying (QPSK). The two
least significant bits (for 16QAM) or four bits (for 64QAM) are used to
carry lower priority service data using QPSK or 16QAM respectively.

[0023] In the hierarchical modulation, the alpha (α) parameter
determines the offset, if any, of the constellation's origin. Depending
on the offsets from the origin of the constellation, the hierarchical
modulation can use uniform (α=1) constellation or non-uniform
(α=2, 3) constellation. Hierarchical modulation with uniform
constellation is using same constellation as normal modulation
constellation but mapping bits to different priority classes. The
Hierarchical modulation with non-uniform constellation offsets the origin
of the constellation. Therefore, with greater offsets of the
constellation from the origin, the distance between symbols in different
quadrants increases so the high priority class robustness increases.
However, the increase in α decreases the distances between
differing symbols within each quadrant which makes it more difficult for
the receiver to differentiate the symbol and thus reduces the robustness
of the low priority class. Consequently, increasing the value of α
increases the robustness of the high priority stream while decreasing the
robustness of the low priority stream.

[0024] In this disclosure, 64QAM with three classes and α=1 is
employed which is shown in FIG. 1. Three classes are used to achieve
finer granularity of block delivery performance and α=1 is selected
in order to make no change with normal modulation. With three classes the
two most significant bits of the 64QAM symbol convey high priority
service data. The two bits in the middle are used to carry medium
priority service data and the two least significant bits are used to
carry lower priority service data.

[0025] FIG. 2 shows an exemplary overview diagram illustrating data
transmission employing the random network coding work with hierarchical
modulation according to the present invention. As illustrated in FIG. 2,
the sender (S) delivers data to the receiver (D) using the hierarchical
modulation (64QAM, α=1) with help of the relays (R1 and R2). The
sender first divides each single packet into a number of small blocks and
encodes the blocks using random network codes. Thereafter, each encoded
block is mapped to one of the High Priority (HP), Medium Priority (MP),
and Low Priority (LP) hierarchical modulation bit positions. Hence, the
coded blocks, y1, y2 and y3 are mapped to the HP, MP, and LP bit
positions respectively. The wireless broadcast nature allows the receiver
and the relays to listen to the transmission of the sender. Depending on
the link quality, receiver and relays detect different number of coded
blocks. The sender (S) transmits in the first time slot, a first relay
(R1) transmits in the second time slot and in the third time slot a
second relay (R2) transmits. In the first time slot, whereas the first
Relay (R1) is able to correctly detect y1, y2, and y3 and the second
relay (R2) detects y1, and y2, the receiver (D) is only able to detect y1
correctly because its link condition is poor due to the long distance
from the sender (S). After successful decoding of the received blocks,
the first relay (R1) generates and transmits different coded blocks, y4,
y5 and y6 using different random linear codes coefficients. In the second
time slot, the second relay (R2) detects y4, y5, and y6 correctly and the
receiver (D) detects y4, and y5. If the decoding is successful using
received, y1, y2, y4, y5 and y6 in the second relay (R2), the second
relay (R2) generates different coded blocks, y7, y8, and y9 and transmits
them. Due to the rateless property of random network codes used across
the blocks in the packet, all the blocks within one packet are equally
useful. After, the receiver may correctly recover the original packet
once it has enough correctly received error free blocks (i.e., "clean
blocks"). Finally, the receiver can recover a packet using any three
correctly received blocks out of detected blocks, y1, y4, y5, y7, y8, and
y9. As number of relay nodes increases, receiver would collect more
"clean blocks" which lead to higher probability of correct decoding.

[0026] The Co-NC (a joint symbol-level random network coding and
hierarchical modulation scheme in a relay communication) has two
advantages over Co-HM. First, in contrast to Co-HM where code rate is
limited to the mother code rate for one encoding block transmission,
Co-NC can decrease the code rate as the number of relays increases since
relays transmit different coded blocks. Second, at each transmission, no
matter where a node resides in a network, there is no need to perform
proper modulation selection procedure to adapt to the link quality,
because the same modulation scheme (hierarchical modulation) is used
every time and small random coded blocks are adaptively transmitted over
different priority classes.

[0028] As depicted in FIG. 3. The encoder divides input bit streams into
certain fixed length segments and adds Cyclic Redundancy Check (CRC)
which is used for error detection at receivers. A CRC appended segment is
called a packet. The sender encodes the packets using random network
coding to generate coded blocks and maps each encoded block to one of the
priority classes bit positions of hierarchical modulation. For exemplary
purpose only, the operation in FIG. 3 is the case where the highest
modulation is 64QAM. The operation with 16QAM and 256QAM hierarchical
modulations is similar to 64QAM hierarchical modulation with differences
of mapping independent coded blocks to different priority classes: 16QAM
with two priority classes, 64QAM with three priority classes, and 256QAM
with four priority classes. Each class of given priority class consists
of two bits. In FIG. 3, the coded blocks are divided into three sets and
mapped to three classes:

y1={y1, y4, y7, . . . }

y2={y2, y5, y8, . . . }

and

Y3={y3, y6, y9, . . . }

[0029] are mapped to the HP, MP, and LP respectively. Symbols modulated
with hierarchical modulation are transmitted to the receiver. The
transmitted symbols can also be received by the relays due to the
broadcast nature of wireless communication. If relays can successfully
decode and recover the transmitted packet, relays generate different
encoded blocks with the packet using different set of random
coefficients. Then, using the same method the sender employed, relays
transmit encoded blocks with hierarchical modulation. Similar to Co-HM
relays, because relays perform decoding and encoding for one packet in
Co-NC, Co-NC relays are required to have same amount of storage as Co-HM
relays. The receiver collects all the encoded blocks transmitted from the
sender and the relays.

[0030] The average of all bits' soft decision values in a coded block is
used to decide whether the coded block is "clean" or not. Using the
average soft decision values of coded blocks, the receiver selects
"clean" blocks for decoding. Even though the probability of error is high
for the coded blocks transmitted in the MP, and LP, there exist some
coded blocks delivered error free among transmitted blocks in the MP and
LP. Since the receiver selects the required number of coded blocks for
decoding from all the received coded blocks, the more coded blocks the
receiver receives the higher the probability of successful decoding it
can achieve.

[0031] Since transmitters can generate and transmit as many coded blocks
as necessary due to the flexibility of random network coding, the number
of coded blocks that relays transmit can be dynamically adjustable
depending on the channel conditions. Therefore, it is possible to
efficiently utilize scarce wireless resources with the proposed scheme.

[0032] The random network code encoder divides each packet into blocks
with a fixed size.

[0033] It is denoted that n as the number of blocks in a single packet,
xi (i=1, 2, . . . , n) as blocks in the packet, and cji (i=1,
2, . . . , n) as the set of random coefficients generated in a given
Galois field, the size of which is determined by the number of bits in a
block (e.g. for a block with 8 bits, GF(28) would be used). A coded
block yj can then be produced as

y j = i = 1 n c ji x i . ##EQU00001##

[0034] Each coded block is a linear combination of all or a subset of the
original data blocks. In this way, the encoder is able to generate a
virtually unlimited number of coded blocks yj (j=1, 2, . . . ) using
different sets of coefficients which are independent of one another, and
any n of these coded blocks can be used to decode by inverting a matrix
of coding coefficients. This is usually referred to as the rateless
property.

[0035] In order to reduce the overhead of communicating random
coefficients between the sender and the receiver for each coded block,
the random coefficient matrix can be pre-generated and kept in the
sender, the receiver, and the relays. In Co-NC, the sender transmits the
index of the pre-generated random coefficients matrix that are used for
encoding to the relays and receiver, as a part of the session control
information before starting to transmit actual data packets. The soft
decision value from the demodulator in the physical layer on the receiver
is used for error detection. Using the average soft decision values of
coded blocks, the receiver constructs a set of blocks to decode from all
the received coded blocks, which always include top n blocks with the
highest average soft decision value. The soft decision values are
estimation of code bit log likelihood ratios (LLRs). In case of perfect
channel knowledge, the estimation of code bit LLR under 2K-QAM can
be obtained by the following equation (1):

[0036] where k is the bit order of used 2K-QAM symbol; ys is the received
QAM symbol; α is the channel gain;

S(sε{s1, s2, . . . , s3k}, s=b1b2
. . . bk)

is the transmitted QAM symbol; σ2 is the variance of noise
which is complex Gaussian random variable with zero mean.

[0037] When choosing a proper coded block size, it may appear that a
smaller block is always preferable, as a smaller block leads to better
delivery rate in the presence of error. Unfortunately, a block that is
too small will lead to an inherent problem that is hard to address. A
block with m bits uses GF(2m) to perform random network coding, and
a smaller number of bits in a block leads to a smaller size of the Galois
Field, with a smaller degree of freedom when coefficient vectors are
randomly chosen. This leads to a higher probability of producing linearly
dependent blocks with random network coding. It is therefore important to
choose an appropriate size for the coded block, so that the block is
sufficiently small, but there is still sufficient degree of freedom to
produce randomized coefficient vectors that are linearly independent of
one another.

[0038] Because enough number of error free blocks is required to recover
the original packet in random network coding, block error rate is more
important than bit error rate. If coded blocks are transmitted using
normal modulation, block error rate is same for all coded blocks.
However, block error rate of coded blocks transmitted over HP bit
positions of hierarchical modulation is lower than block error rate of
coded blocks transmitted over LP bit positions of hierarchical
modulation. Therefore, overall block error rate can be decreased and
decreased block error rate eventually lowers packet error rate.

[0039] FIG. 4 shows a block error rate and packet error rate comparison
between random network coding with hierarchical modulation and normal
modulation. In fact, the simulation results for Additive White Gaussian
Noise (AWGN) channel between a sender and a receiver are shown in FIG. 4.
The benefit of jointly using hierarchical modulation with random network
coding is more substantial when operating range is low SNR region which
is the case with low code rate. As SNR gets lower, block error rate using
hierarchical modulation becomes lower than the one using normal
modulation. Because low code rate of random network coding can be
achieved by increasing number of relays, the proposed scheme of jointly
using hierarchical modulation with random network coding perfectly fits
for relay communication.

[0040] The other benefit of joint use of hierarchical modulation with
random network coding is simplicity of modulation scheme selection. In
order to meet required block error rate with normal modulation, proper
modulation scheme has to be selected. However, modulation scheme can be
fixed with hierarchical modulation and simple coded block mapping to
different priority class can guarantee certain block error rate. This
scheme is extremely useful when there are relays with different channel
conditions. Relays can adaptively collect error free coded blocks.

[0041] To evaluate the performance, the PER performance of Co-NC with
Co-HM for the same size packet have been compared. Since a main concept
of present disclosure is a random network coding with hierarchical
modulation in relay communication, PER performance and delay performance
of random network coding with normal modulation in single hop
transmission is omitted.

[0042] With respect to the Co-HM, convolutional codes of rate 1/2 with
well known soft combining and soft output Viterbi decoding algorithm are
used. Also, "decode-and-then-relay" strategy for each intermediate nodes
for Co-HM is used. For simplicity, the 16QAM hierarchical modulation for
both the Co-NC and the Co-HM are used in the evaluation. For fair
comparison with the Co-HM, in the Co-NC simulation, the source transmits
2n number of coded blocks to make the rate 1/2 when the original packet
is divided into n number of blocks and the relays, which have
successfully decoded the original packet, transmit new n number of coded
blocks. It is assumed that a path-loss exponent is 3.52 and that two
relays are located between the sender and the destination with a distance
d. FIG. 5 shows a network topology used in a simulation. Each relay is
located along the middle point d/2 so that the distance from the relay to
the sender and to the destination equals 2d/3, forming a diamond shape
where each edge is 2d/3 long. It is performed that the simulation based
on the scenario where two protocols transfer a large file over AWGN
channel between a sender and a receiver with help of one or two relays. A
file is divided into segments with 16 bit CRC appended to each segment. A
segment with CRC is called a packet. In the simulation, a packet size of
512 bits is simulated. Each packet is divided into a number of blocks, on
which random network coding is performed. A block size of 8 bits is used
which is chosen based on the extensive simulation and achieves the best
performance.

[0043] The simulation results are shown in FIG. 6. When there is no
cooperation (direct link from the sender to the receiver), the Co-NC
shows worse performance than the Co-HM for low Signal-to-Noise-Ratio
(SNR) values because random network coding does not provide error
correction capability for corrupted bits. However, the Co-NC shows better
performance than the Co-HM as SNR gets higher because the Co-NC uses
"clean" coded blocks selected from all the received coded blocks either
through the HP or LP in decoding. The one relay case, where one relay is
helping the packet transmission, shows similar performance trend to the
direct case. The Co-HM only benefits very little from the inclusion of
the second relay. That is because the code rate of the Co-HM cannot be
decreased less than the mother code rate used for encoding of the
transmitted block in the sender. The gain mainly comes from the soft
combining of blocks received from two relays. The Co-NC outperforms the
Co-HM as the number of relays grows due to the rateless property of
random network coding. The proposed relay communication scheme with joint
symbol-level random network coding and hierarchical modulation achieves
approximately 2.1 dB, and 3.9 dB gain at PER of 10?2 with one relay and
two relays respectively as compared to the conventional coded cooperation
scheme.

[0044] In this disclosure, the use of network coding to improve relay
communication using hierarchical modulation has been explored. The joint
use of random network coding and hierarchical modulation makes modulation
selection scheme simple and relay communication more effective with
flexible rate adjustment regardless of link quality. Using the proposed
scheme, the communication scheme of multi-hop relay consumer device
networks is expected to be simplified substantially which will eventually
reduce implementation cost. Simulation results with AWGN channels showed
that the proposed system can outperform the conventional coded
cooperation scheme with hierarchical modulation.

[0045] The present invention may provide a method for transmitting data in
wireless communication system, the method comprising: dividing an input
bit stream into segments; adding error detection code into each segments
of the divided input bit stream in order to generate a packet; dividing
the generated packet into a plurality of blocks with fixed size; coding
each of the plurality of blocks using a random linear coding in order to
generate coded block bits; mapping the coded block bits to one of a
hierarchical modulation bit positions, wherein the hierarchical
modulation bit positions are a group of bits representing a priority
class; and transmitting the plurality of modulated symbols, wherein the
priority class is at least one of a high priority, a medium priority, and
a low priority, the all steps are implemented in a multi-hop relay
networks, and the coded block bits in a coded block are rearranged
according to a bit error probability.

[0046] It can be also said that the present invention may provide a method
for receiving data in wireless communication system, the method
comprising: receiving a plurality of modulated symbols; generating coded
block bits by selecting bits from the hierarchical modulation bit
positions of the received plurality of modulated symbols, wherein the
hierarchical modulation bit positions is a group of bit representing the
priority class; and performing a random linear decoding with the coded
blocks to restore a packet, wherein the priority class is at least one of
a high priority, a medium priority, and a low priority, and the all steps
are implemented in a multi-hop relay networks.

[0047] Although the present disclosure is described in the context of
mobile communications, the present disclosure may also be used in any
wireless communication systems using mobile devices, such as PDAs and
laptop computers equipped with wireless communication capabilities (i.e.
interface). Moreover, the use of certain terms to describe the present
disclosure is not intended to limit the scope of the present disclosure
to a certain type of wireless communication system. The present
disclosure is also applicable to other wireless communication systems
using different air interfaces and/or physical layers, for example, TDMA,
CDMA, FDMA, WCDMA, OFDM, EV-DO, Wi-Max, Wi-Bro, etc.

[0049] Code in the computer readable medium may be accessed and executed
by a processor. The code in which exemplary embodiments are implemented
may further be accessible through a transmission media or from a file
server over a network. In such cases, the article of manufacture in which
the code is implemented may comprise a transmission media, such as a
network transmission line, wireless transmission media, signals
propagating through space, radio waves, infrared signals, etc. Of course,
those skilled in the art will recognize that many modifications may be
made to this configuration without departing from the scope of the
present disclosure, and that the article of manufacture may comprise any
information bearing medium known in the art.

[0050] As the present disclosure may be embodied in several forms without
departing from the spirit or essential characteristics thereof, it should
also be understood that the above-described embodiments are not limited
by any of the details of the foregoing description, unless otherwise
specified, but rather should be construed broadly within its spirit and
scope as defined in the appended claims, and therefore all changes and
modifications that fall within the metes and bounds of the claims, or
equivalents of such metes and bounds are therefore intended to be
embraced by the appended claims.

Patent applications by Yong Ho Kim, Gyeonggi-Do KR

Patent applications in class For packet or frame multiplexed data

Patent applications in all subclasses For packet or frame multiplexed data